(Monday, 7/15/2019) 1:15 PM - 1:45 PM
Room: Exhibit Hall | Forum 1
Purpose: The MR-linac (MRL) provides image guidance with high soft-tissue contrast. Esophageal cancer patients are a potential group to benefit from better guidance in the MRL. However, due to the presence of a transverse magnetic field (1.5T), electron deflection may affect the target dose distribution, particularly at tissue interfaces. The purpose of the study is to compare radiotherapy treatments plans in esophageal cancer calculated for a MRL with plans for a conventional linac.
Methods: We prospectively enrolled 20 cases with ESCC into a cohort study which underwent re-planned using the research version of Monaco (V 5.40.00, Elekta AB, Stockholm, Sweden). All patients received 4DCT and three dimensional (3D) CT scans before RT. Intensity modulated radiotherapy (IMRT) plans were designed on 3DCT images and 0% phase of the 4DCT images for the Elekta MRL and a conventional Elekta linac, respectively. The prescribed dose was 60 Gy to the primary tumor (PTV60) and the esophageal wall contralateral (CE) to the tumor was contoured as an avoidance structure. All generated treatment plans were optimized for fulfilling their respective planning constraints.
Results: The dose recalculation showed the presence of the 1.5 T magnetic field can slightly reduce PTV dose (D95). The low dose areas (V5, V10) of the lung sites are higher than the conventional linac. The MRL results in considerable increases in Dmax on the esophageal wall in all cases. No significant differences between both plans were seen in terms of V20Gy of the lungs and V40Gy of the heart.
Conclusion: The treatment of patients in the MRL using a conventional IMRT technique, at 1.5 T, implies an increase in esophageal wall and V5, V10 of the lung sites dose. The dosimetric impact of magnetic field at tissue-air boundaries is minor and does not negatively impact target conformity index. This opens possibilities for developing MR-guided esophageal cancer treatments in the MRL.
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